Gridfinity 2x5 6u height.
Holds 7 spools with a max diameter of 27mm.
Stackable. Could have made it shorter but I wanted it to be the same height as the other bins on the same grid.

For more spools that will fit (as I wanted to organize the random stringy bits in my craft cabinet), I printed this spool and scaled the diameter to 27mm in Bambu Studio.
https://www.printables.com/model/345591-sewing-machine-thread-spool